The Effects of Assistive Force on Size and Irregularity of Muscle Force Variability during Mechanically Assisted Force Coordination

Objective: This study aims to observe how force output patterns alter when assistive force is provided during mechanically assisted force coordination with isometric elbow flexion.<br/>Background: Force coordination between humans and assistive robots requires human muscle force to be released in response to mechanical assistance. However, little is known about how motor performance of muscle force release with assistive force is different from the one without assistance. This study investigated how force variability parameters of coefficient of variation (CV) of force output, sample entropy, and stabilization time alter during isometric force control with assistive force. <br/>Method: Eleven male participants (mean: 24.4; SD: 1.5) participated in this experiment. Electromyography electrodes were placed on the surfaces of biceps and triceps muscles. The participant maintained an isometric elbow flexion force of 47N with the dominant arm along a linear trajectory shown on a screen. Under the conditions of assistive force provision, the assistive force was gradually provided as 25% or 50% of the isometric force for 0.5s by operating the actuator so that the participant could release their muscle force. Under the conditions where assistive force was not provided, the participant released their isometric force by 25% or 50% along with the visual guidance on the screen. From the manual force output, release stabilization time of force control was calculated and the CV and sample entropy were computed to represent size of force variability and time-series data irregularity, respectively.<br/>Results: Regardless of whether assistance was provided, the activation of biceps and triceps brachii muscles decreased after force release. This study found that compared to the visually guided conditions, mechanically assisted conditions showed a lower CV, longer stabilization time, and higher sample entropy. When the muscle force was greatly released (50% release of the isometric force), a longer stabilization time and greater CV were observed, but the entropy was lower compared to the slight muscle force release (25% release).<br/>Conclusion: This study suggests that when the magnitude of force release is relatively large, the size of force variability increases, while the irregularity decreases. This could imply that humans probably have potential adaptability for greater assistive force provided during mechanically assisted force coordination.<br/>Application: The results of the current study can be used to determine the amount of assistive force provided during human-robot collaboration.